Fuel cells generate electric power and heat simultaneously through an electrical chemical reaction between a fuel gas containing hydrogen and an oxidizing gas containing oxygen, such as air. The fuel cells are classified into various kinds according to a fuel or material used. One example is a polymer electrolyte fuel cell using a polymer electrolyte membrane. FIG. 11 is an enlarged view of an end portion of a conventional polymer electrolyte fuel cell 51. The polymer electrolyte fuel cell 51 includes a cell stack 53 in which plural cells 52 each containing the polymer electrolyte membrane are stacked, current collectors 54 are provided at both ends of the cell stack 53, and end plates 55 are provided outside the current collectors 54. They are fastened by tightening them from both sides by bolts. A reaction gas supply inlet 56 is provided at an end surface of the fuel cell 51 to supply reaction gases (fuel gas and oxidizing gas) required for power generation and an external pipe P is connected to the reaction gas supply inlet 56 to feed the reaction gases.
The polymer electrolyte membrane included in the polymer electrolyte fuel cell 51 must always maintain a wet state to keep ion conductivity. Typically, at least one of the fuel gas and the oxidizing gas (hereinafter these are referred to as reaction gases) which contact the polymer electrolyte membrane is humidified and then is supplied to the fuel cell 51. In this case, the reaction gas is humidified to a state which is close to a saturated state. Therefore, performance degradation phenomenon called “flooding” occurs, in which if the temperature of the pipe in a path is lower than the temperature of the reaction gas, water condensation occurs, impeding supply of the reaction gas and reducing a power generation voltage.
By winding a heat-insulating material around the external pipe P, the water condensation within the pipe can be prevented. However, the heat insulating material cannot be wound around a portion inside the fuel cell 51, and consequently, water condensation may occur depending on the use condition. Basically, at the start-up of the fuel cell 51, the temperature of the interior (i.e., cell stack 53) is increased up to 60 to 90 degrees centigrade. This may possibly avoid the water condensation inside the fuel cell 51. Actually, the end plate 55 which is located at an outermost side and has a large thickness cannot increase in temperature according to an temperature increase in the interior of the fuel cell (cell stack 53) and a low-temperature state continues immediately after the start-up. For this reason, water condensation occurs inside a portion of a path in the vicinity of the end plate 55.
Under the circumstance, in order to prevent water condensation in the vicinity of the end plate, a fuel cell is proposed, having a structure for connecting an external pipe to a cell stack via a joint, instead of connecting the external pipe to the end plate (e.g., see FIG. 3 in patent document 1). FIG. 12 is an enlarged view of an end surface portion of a fuel cell 61 having the above described structure, in which FIG. 12(a) is a cross-sectional view and FIG. 12(b) is a perspective view. As shown in FIG. 12, the fuel cell 61 includes a joint 63 connecting a cell stack 62 to an external pipe P. A current collector 64 and an end plate 65 have a through-hole 66 and a through-hole 67, respectively, which have a larger diameter than the joint 63 so that the joint 63 does not contact the end plate 65. In such a structure, since the joint 63 does not contact the end plate 65, it is possible to prevent water condensation within the reaction gas path (within the joint 63) which may be caused by contact with the end plate 65.    Patent document 1: Japanese Laid-Open Patent Application Publication No. Hei. 7-282836